FR0105753 ROLE OF MEMBRANES AND MEMBRANE REACTORS IN THE HYDROGEN SUPPLY OF FUEL CELLS FOR TRANSPORTS

Anne JULBE. Christian GU1ZARD Laboratoires des Materiaux et des Precedes Membranaires CNRS-UMR 5635 Institut Europeen des Membranes - UMII - Place Eugene Bataillon - 34095 Montpellier cedex 5 - France Mel : [email protected]

Abstract: Production, storage and supply of high-purity hydrogen as a clean and efficient fuel is central to fuel cells technology, in particular in vehicle traction. Actually, technologies for handling liquified or gaseous hydrogen in transports are not available so that a number of alternative fuels are considered with the aim of in-situ generation of hydrogen through catalytic processes. The integrated concept of membrane reactors (MRs) can greatly benefit to these technologies. Particular emphasis is put on inorganic membranes and their role in MRs performance for H2 production. Key words : membrane reactors, inorganic membranes, .

1. INTRODUCTION between catalysis, membrane science and chemical engineering. In such an integrated process, the membrane is High-purity hydrogen is required for fuel operation. used as an active participant in a chemical transformation Regarding transportation applications, of the many for increasing the reaction rate, selectivity and yield. problems to put the fuel cell devices to practical use, two The concept of combining membranes and reactors is major problems must be pointed out. First a large amount being explored in various configurations which can be of gaseous or liquified hydrogen is not easy to carry on a classified in three groups, related to the role of the vehicle, second the CO concentration in the hydrogen fuel membrane in the process [1], Of course the following stream should be controlled at a very low value, usually considerations are very schematic and many intermediate, less than 20 ppm for polymer electrolyte type fuel cell. derived or coupled complex systems exist. As shown Instead of carrying hydrogen, fuels like , below, the membrane can act as: , naphta oil ... can be interesting alternatives - an extractor: the removal of product(s) increases the provided the utilization of a conversion process operated in reaction conversion by shifting the reaction equilibrium, combination with the fuel cell to produce the high-purity hydrogen required. Shift of reaction equilibrium In other respects the interest of membrane reactors (MRs) has been largely demonstrated at the laboratory scale, A + B ^ ^ C + D namely for hydrogenation, dehydrogenation, oxidation and decomposition reactions including partial oxidation and Increased conversion oxidative coupling of . In particular, catalytic reforming and partial oxidation are two reactions for which the concept of integrated process developed with MRs can be advantageously applied for the production of hydrogen. - a distributor: the controlled addition of reactant(s) limits Apart from electrocatalytic reaction in direct methanol fuel side reactions, cells which work at lower temperature, high temperature and chemical harsh environment are generally encountered A C A C Controlled addition of a for most of the aforementioned reactions. These two factors reactant. Limitation of side strongly favor inorganic membranes for MRs applications. reactions Consequently, the membranes themselves must be B B mentioned as an important factor directing further A -----► B (-»D) commercial availability of these systems as far as they still need optimization and new developments. Increased selectivity The scope of this paper is to briefly address the present status of MRs in relation to hydrogen production but also - an active contactor: the controlled diffusion of reactant(s) the actual limitations of this technology and how it can be to the catalyst layer can lead to an engineered catalytic improved in order to meet requirements of fuel cell zone. operation. Particular emphasis is put on inorganic membranes and their role in MRs performance. A A + B Controlled diffusion of reactants to the catalyst 2. BASIC CONFIGURATIONS OF Cat. MEMBRANE REACTORS A + B—► C

MRs as a concept, dates back to 1960s. Many research Increased conversion efforts have been devoted to this subject at the frontier (and selectivity) In the two first cases the membrane is usually considered liquid film, as far as gas/liquid interface is created inside as catalvtically inert and is coupled with a conventional the pores in direct contact with the catalyst [1,8]. fixed bed of catalyst placed on one of the membrane sides. In addition to the aforementioned MRs configurations, In the case of reactor design based on an electrochemical the main components of this technology, membranes and cell configuration, the catalyst is coated on one of the catalysts, can be combined in the reactor following three electrodes or can serve as an electrode itself. The extractor main arrangements (figure 1): the catalyst is physically mode corresponds to the earlier applications of MRs and separated from the membrane, the catalyst is dispersed in has been applied to increase the conversion of a number of the membrane, or the membrane is inherently catalytic. For equilibrium limited reactions, such as alkane the former case, the term “inert membrane reactor” (IMR) dehydrogenation, by selectively extracting the hydrogen has been proposed by opposition to the two other ones produced. Other hydrogen producing reactions such as the which are “catalytic membrane reactors” (CMR). This gas shift, the steam reforming of methane and the means that the membrane has to be adapted to both MRs decomposition of H2S and HI, have been successfully configurations and membrane/catalyst arrangements. investigated with the MR extractor mode [2], The hydrogen permselectivity of the membrane and its permeability are two important factors controlling the efficiency of the z____ Catalyst pellets process. The distributor mode is typically adapted to (or layer) consecutive parallel reaction systems such as partial -<------Inert membrane oxidation or oxydehydrogenation of hydrocarbons or oxidative coupling of methane. For these applications the <------Porous support membrane, separating the alkane from , is generally used to control the supply of oxygen in a fixed bed of (a) catalyst in order to bypass the flammability area, to optimize the oxygen profile concentration along the ------Catalyst dispersed reactor, and to maximize the selectivity in the desired FFF in the membrane oxygenate product. This concept has also a beneficial role in mitigating the temperature rise in exothermic reactions (b) [3], In such reactors, the utilization of mixed ionic/electronic conductive (MIEC) membranes is an Inherently catalytic important economic factor because air can be feed instead membrane of pure oxygen. However, the limited permeability of dense oxygen permselective membranes below 800°C and problems of long term stability has limited their development, although such membranes are now commercialized [4], The higher, stable and controllable permeability of porous membranes is then also considered Figure 1 : The three main membrane/catalyst as attractive for a number of oxidative reactions [5]. arrangements in membrane reactors, a) inert membrane in In the active contactor mode, the membrane acts as a contact with the catalyst; b) membrane/catalyst composite diffusion barrier and does not need to be permselective but material; c) membrane with inherent catalyst properties. catalytically active. The concept can be used with a forced flow mode or with an opposing reactant mode. The forced flow contactor mode, largely investigated for 3. MEMBRANE REACTORS AS A WAY OF catalyzed reactions [2], has been also applied to the total MAXIMAZING HYDROGEN PRODUCTION oxidation of volatile organic compounds [6] or the alkene hydrogenation triphasic reactions [7], One of the The absence of objectionable combustion products makes advantages of this forced flow mode is to prevent back- hydrogen the most important clean fuel cell operation, in mixing of the initial products. The opposing reactant particular in transportation applications. A number of contactor mode applies to both equilibrium and irreversible chemical processes exist which can be used in view of reactions [8], as far as the reaction is sufficiently fast producing hydrogen from the most abundant fossil fuels, compared to transport resistance (diffusion rates of namely coal, petroleum, natural gas, and biogases [9]. reactants in the membrane). In such a case, a small reaction However, natural gas and petroleum liquids contain sulphur zone forms in the membrane (if sufficiently thick and derived organic compounds that normally have to be symmetric) in whicn reactants are in stoechiometric ratio. removed before any further fuel processing. This aspect This concept has been demonstrated experimentally for will not be treated here but requires a careful design to reactions requiring strict stoechiometric feeds such as the ensure low sulphur levels in the fuel (typically below 0.1 Claus reaction or for kinetically fast strongly exothermic ppm). heterogeneous reactions such as partial oxidation [8], Triphasic (gas/liquid/solid) reactions which are limited by 3.1. Chemical reactions involved in the conversion of the diffusion of the volatile reactant (e.g. olefin fossil fuels to hydrogen. hydrogenation) can also be improved by using this concept. Catalytic steam reforming is a mature technology, operated Indeed the reactant does not have to diffuse through a industrially on a large scale for hydrogen production. The

56 Piles a combustible et interfaces pour les transports basic endothermic reforming reaction for methane is fuel cell supply. They are regarded here from the carried out above 500°C: standpoint of their catalytic processing in MRs.

(1) CH, + H20 ->CO + 3Hi AH = 206 kJ.mol" 1 Reactor and membrane A housing tubes with the associated exothermic water gas shift reaction:

(2) CO + HiO -o- C02 +H2 AH = -41 kJ.mol' CH, mm+y ~ ~ <=> ( (>: '2U- W////A The COi reforming, or dry reforming, can be carried out if Air there is no ready source of steam: outlet

(3) CH4 + COi 2CO + 2 Hi AH = 247 kJ.mol ' CH, 2 2 \ 1 ■.■'fr C02 +2H 2 Methane is not the only fuel suitable for steam reforming. Methanol can also be converted at lower temperature, Hot-seal MIEC membrane typically at 250°C, according to the endothermic reaction: Figure 2 : Schematic representation of a membrane (4) CH3OH+ HiO -> C02 +3H2 AH = 49.7 kJ.mol" 1 reactor operating with a tubular MIEC membrane for production [4], It is possible to provide the heat required to sustain these endothermic reactions from the electrochemical reaction in The partial oxidation of methane to produce syngas the stack of fuel cells. The so-called concept of internal (reaction 7) has been the most investigated reaction in reforming is applied to the molten carbonate and solid conjunction with MRs. In other respects, due to the oxide fuel cells, on account of their high operating possibility for ceramic ion conductive membranes to temperatures. One of the most critical issues when deliver oxygen directly from air, two types of membrane operating these reactions is the risk of carbon formation on reactors have been investigated : pressure driven devices metal catalysts according to the side reactions: with mixed ions-electrons conducting (MIEC) membranes or electrically driven devices with ionic conductor oxide- based membranes. In the technology proposed by Argonne (5) CH, -» C + 2 H% National Laboratory in the US [4], MIEC membranes were (6) 2CO-»C + C02 implemented in a membrane reactor for syngas production. This kind of reactor can be operated with a tubular Fortunately, it is possible to reduce such carbon formation membrane according to the general configuration shown in by adding excess steam to the fuel feed. figure 2. It has been successfully operated for more than As an alternative to steam reforming, methane and other 1000 h at 800-900°C. The membrane tube based on the Sr- hydrocarbons may be converted to hydrogen for fuel cells Fe-Co-O system was observed to be stable even with via partial oxidation: highly reducing methane and highly oxidizing air environments on either side of the membrane. (7) CH, + 02-> C02 + 2 H2 AH = -319 kJ.mol" 1 Electrolyte This highly exothermic reaction occurs at a very high (YSZ) temperature (typically 1200 to 1500°C) in absence of a catalyst. So far, platinum or nickel metal are used as Anode Cathode catalysts to reduce the reaction temperature. compartment compartment Autothermal reforming usually describes a process in ch4 Air which both steam and air are fed with the fuel to a catalytic reactor. The advantages of autothermal reforming is a simple system design in which all the heat for the reforming reaction is provided by partial combustion of the fuel and in which less steam is required compared with conventional reforming. Finally CO clean up required for some of the fuel cells (namely PAFC and PEM fuel cells) is usually achieved by using chemical processes such as selective oxidation or Ni/catalyst methanation.

3.2. Recent developments in membrane reactors applied to hydrogen production Methane or natural gas as well as methanol are considered as very promising fuels for producing hydrogen in view of Figure 3 : Schematic representation of an electrochemical reactor for syngas production [11].

Piles a combustible et interfaces pour Ies transports 57 An electrochemical membrane reactor for the partial should be also noted that in such a reactor configuration, oxidation of hydrocarbons has been described as an other unreacted products can migrate from one reactor to another possible utilization of MIEC-based membrane reactors and are present in the hydrogen stream produced. This is [10]. In fact MIEC membranes serve as electrodes in the detrimental to the process because a too high concentration electrocatalytic device. The same concept based on solid of CO and CH4 in the hydrogen stream is not allowed for oxide electrolytes has also been proposed as an efficient, fuel cell operation. economical and simplified methane conversion process, As speculated hereafter, the concept of integrated compared with the conventional steam reforming way [11]. membrane reactor combining distribution and extraction Basic operation principle of this electrochemical cell is modes for the membranes could solve the problem of shown in figure 3. In this system, syngas production is unreacted product migration. As shown in figure 5, instead achieved at 900°C at the anode which is coated with the of a two reactor setup, three membranes can be used in the appropriate catalyst. same reactor with a specific role for each of them : A possible enhancement of hydrogen production has been - a mixed ionic/electronic conductive membrane for recently proposed [12] using a catalytic process combining oxygen distribution to the syngas compartment partial oxidation of methane and water gas shift reaction. - a microporous membrane separating the syngas and the For this purpose, a two-reactor system (figure 4) has been water gas shift compartments with a double role of designed in which a ceramic membrane is used as oxygen extractor/distributor for CO and barrier for CH4 , distributor in a first reactor for the conversion of methane - the third membrane with an extractor role for hydrogen is to syngas (H2 + CO) at 850-900°C. Hydrogen production is at the outer part of the water gas shift compartment. The enhanced in a second catalytic reactor through the water membrane has to prevent CO migration in the produced gas shift reaction (2), in which CO reacts with the steam hydrogen stream and let the C02 permeates or not injected into the reactor at a controlled rate to produce C02 depending on its characteristics. and H2. This combination of syngas production and water In this concept, it is assumed that there is no resistance to gas shift reaction resulted in a ratio H2/CH4 (conlc,1ed) > 2.9. hydrogen permeation from the syngas compartment to the WGS one. air

o2 from air Ceramic Membrane Reactor -=> T = 800-900°C Rh Catalyst <=$ Catalyst comnartments Syngas Reaction ■=> CH4 + % 02 -> CO + 2H2 <— C02 , Hj selective membrane CO, H2 selective membrane mixed ionic/electronic conductive membrane WGS Reactor for oxygen transport h2o T = 250-450°C Cu/Zn/AljOj Catalyst ■=> Water Gas Shift Reaction Syngas reaction CO + h2o -> C02 + If Z3 CH4 + V2O2 = CO + 2H : Water Gas Shift Reaction CO + H2 0 = COz + 2H 2

Figure 5 : Speculated'arrangement of membranes in an integrated membrane reactorconcept coupling syngas and water gas shift reactions for hydrogen production

Figure 4 : Schematic diagram of a two-reactor setup for As a liquid fuel for producing hydrogen, methanol has maximizing hydrogen production [12J. advantages relative to other hydrocarbons because of its low steam to carbon ratio, relatively low reforming As it can be seen from this new approach, combining a temperatures (250-350°C), high quality (sulphur < 5ppm) catalytic membrane reactor for the partial oxidation of and ease of handling. Experimental MR setups have been methane with a conventional reactor for the water gas shift described in the literature for methanol steam reforming can significantly improve the hydrogen production. It (reaction 4). Most of them are based on Pd-derived

58 Piles a combustible et interfaces pour les transports membranes for hydrogen extraction. A recent work [13] substituted by aliovalent cations have been mentioned as presents a new design of double jacketed membrane reactor excellent mixed conductors but have not yet been tested as which is set up to perform the steam reforming and membrane in IMRs. Ceria and ceria-based oxides have oxidation reactions at the same time. The concentric collected much attention as alternative oxides to zirconia. module consists of a supported Pd membrane tube located In fact, the magnitude of electrical conductivity and the at the center position and two stainless steel tubes stability under reductive atmospheres of these compounds separately assembled as double outer jackets. A Cu-based are greatly dependent on the type and quantity of doping catalyst and Pd/Al203 catalyst are used respectively for the elements. For example, the electrical conductivity of steam reforming of methanol and the oxidation of gases gadolinia-stabilized ceria has been found to be about one rejected from the membrane tube. Methanol and water are order magnitude larger (10"‘ S.cm"1 at 800°C) than for YSZ fed into the reforming catalyst to proceed the reaction at electrolytes. A number of ceria-based oxides can be 350°C and 6-15 atm. It has been noted that increasing the reduced and electronic conduction becomes significant at reaction pressure increases the hydrogen flux and the low oxygen partial pressures. recovery yield. A recovery yield up to 97% with a Perovskite-type compounds have been described in the hydrogen flux of 3.7nf m"2h"' was claimed for this MR literature as the most important category of M1EC that can application. This is a much higher recovery yield than operate for oxygen permeation without electrodes or conventional methods. external electrical circuitry. A large number of these One can see that intrinsic properties of the membranes compounds have been mentioned as very promising and are play a central role in the design of MR configurations. still under investigation. The (La,Sr)(Co,Fe)03.5 oxides Selectivity, permeability, reliability, chemical and thermal have been pointed out as one of the most interesting resistances, are as many compulsory characteristics for a perovskite series for fuel cells or MRs applications. More sustainable development of these technologies. In the recently LaGa03-based perovskites were found to exibit following we shortly review the present status of inorganic efficient conductivity, comparable with that of Ce02-based membranes with potential applications in MRs. oxides. Another category of compounds, Bi4 V2On with intergrowth structures has been identified as leading to fast 4. ACTUAL AND EXPECTED MEMBRANE O2" ion conduction at T < 400°C. They are currently PERFORMANCE FOR HYDROGEN PRODUCTION experienced as electrolytes for a new generation of oxygen pumps able to work at intermediate temperatures. As Two main categories of membrane are of interest in already mentioned, SrFeCo05 Ox, a non-perovskite catalytic processes dedicated to hydrogen production : compound designated as SFC-2, has been successfully used those involved in the reaction process (contactor) and the by Argonne National Laboratory to develop a small ones able to assist the reaction by supplying reactants prototype based on the MR concept for syngas production (distributor) or removing products (extractor). Today, the through partial oxidation of methane. The recently latter category can be considered close to industrial commercialized dense MIEC membranes (Eltron research application whereas investigations on the former are still at Inc. [4]) show that mixed ionic/electronic conductive the laboratory scale. Only characteristics of distributor and membranes could became competitive candidates for extractor membranes will be developed here. Present status partial oxidation of methane in a membrane reactor. of inorganic membrane contactors can be found in a recent However, though they are still promising, mixed review on porous ceramic membranes for catalytic reactors ionic/electronic conductive membranes prepared from [14]. these categories of compounds did not entirely satisfy until now the requirements of high ion conduction at low 4.1. Inorganic membranes involved in catalytic temperature and/or long-term stability under a reduced processes for hydrogen production oxygen pressure. In recent years, membranes acting as oxygen distributors in Membranes for hydrogen transport constitute an other steam reforming or partial oxidation of alkanes has arose as category which plays an important role in the design of an important issue for hydrogen production and fuel cell membrane reactors [17], in particular for syngas production operation. They can be dense or porous [15]. Ceramic ion and hydrogen recovery. Due to the considerable conductive membranes (CICM) exclusively permselective of hydrogen in pure palladium (Pd adsorbs 600 times its to oxygen are very promising for industrial applications volume of H2 at room temperature); membranes for [16]. They generally outperform the porous systems if their hydrogen purification based on Pd-alloys have not been thickness is sufficiently low; mainly because they are able surpassed so far. Self-supported Pd membranes are too to deliver pure oxygen directly from air. thick (typically 50 pm or greater) resulting in low hydrogen Thin films of fluorite-based ion conductive oxides can be flux which inhibits their application for membrane reactor used as oxygen distributors in an electrochemical use. Consequently developments have concentrated on the membrane reactor for methane conversion as described in production of composite membranes composed of thin figure 3. Zrl.xYx02x/2 (YSZ) has been the most investigated layers of Pd or Pd alloys deposited onto a porous substrate material and it is considered as the most reliable candidate such as ceramic or stainless Steel. The role of Pd-based so far. Nevertheless, the limiting magnitude of electrical membranes is mainly to remove hydrogen (extractor) from conductivity and the high operating temperature (900- the catalytic reaction source. For equilibrium limited 1000°C) required create serious technological problems in reactions it is important to have a high permeation rate to terms of stability of the different components of the reactor. cope with the usual high activity of the catalyst. Hydrogen Compounds in the system 5-Bi 203, in which Bi is flux can be enhanced by an increase of the hydrogen partial

Piles a combustible et interfaces pour les transports 59 pressure gradient across the membrane. This is usually applications, porous infiltrated composite membranes are achieved by applying a vacuum to the permeate side or by attractive candidates. Indeed these membranes in which the applying a sweep gas. One problem often cited is the effect membrane material is deposited inside the pores of a of poisons such as H2S or CO on Pd membranes. It is clear porous robust support (figures 7b), exhibit a good that H2S can have a deleterious effect under appropriate thermochemical resistance, a low sensitivity to the presence conditions, but the effect of other gaseous contaminants is of defects, a barrier effect, and are easily reproducible. less clear. It seems from tests of adding up to 10% of CO, Furthermore, in the case of catalytic membranes, a C02 and H20 to the permeating stream that addition of C02 relatively high quantity of catalyst can be deposited in such has only a marginal effect whereas both CO and H20 can a membrane configuration. cause a severe reduction in hydrogen permeation rate, with steam being the most serious causing reductions of 30-40% in the permeation rate. This is a strong limitation for membranes involved in the design of MRs for water gas shift reaction (see figure 5). However, poisoning effect of CO and H20 is temporary; removal of the poison additive enables the membrane to retain its original permeability. In the next future mixed proton-electron conducting membranes could became alternative candidates to Pd membranes for H2 selective transport [18]. Microporous carbon, zeolite or silica based membranes can be also considered for hydrogen permeation in MRs [14]. Their selectivity for hydrogen extraction is not so high than with palladium membranes but in some cases they offer a better chemical and temperature resistance. A b) Composite infiltrated membrane I recent paper [19] describes the preparation of a silica membrane which exhibits hydrogen selectivities of 100% with respect to CH4 , CO, C02 and H20. The membrane revealed to be stable to hydrothermal stresses (10%H2O at 500°C and 1 bar) over 150h of operation. The membrane was employed in a catalytic reactor for dry reforming of methane (reaction 3), using a 1% Rh/Al203 catalyst. As a result of simultaneous reaction and separation, conversions were higher than those obtained in packed-bed reactor operated at the same conditions, and exceeded equilibrium levels. Figure 7 : Schematic representation of an asymmetric thin 4.2. New membrane concepts for MRs film supported membrane (a) and a composite infiltrated Roles of membrane function(s), membrane/catalyst membrane (b) [14], arrangements and membrane structure in MRs are now clearly established. Recent progress in MRs technologies A number of synthesis method have been experienced can be certainly attributed to chemical engineering and described in the literature as being well adapted to the contributions. Nevertheless, improvements of the preparation of infiltrated membranes. In our case, the sol- membrane itself can be beneficial to future developments gel process and the solvothermal synthesis have been of MRs, in particular for hydrogen supply to fuel cells. successfully applied to infiltrated membrane preparation. New membrane materials and membrane concepts have An example of a sol-gel derived infiltrated membrane is been recently developed in our laboratory which show how given in the next paragraph. Regarding solvothermal material science can contribute to MRs technologies. synthesis, MFI and V-MFI zeolite infiltrated membranes have been synthesized in a porous alumina support and 4.2.1. Infiltrated membrane materials for more used as membrane contactors [20]. Such composite reliable membranes membranes are stable up to 650°C and withstand numerous The major problem for thin film supported inorganic heating/cooling cycles. membranes (figure 7a) is the presence of defects induced during the different preparation steps. Defects are 4.2.2. The innovative concept of chemical valve detrimental to selectivity and, as it has been mentioned membrane for oxygen distribution above, they render the membranes non-operant in gas In MRs for consecutive parallel reaction systems such as phase catalytic processes, specially for pure hydrogen partial oxidation and oxydehydrogenation of hydrocarbons production. The development of simple and reliable or oxidative coupling of methane, the membrane acts as a synthesis methods, easily adaptable to large scale distributor separating the alkane from 02. Its role is production of inorganic membranes is one of the keys of generally to control the supply of 02 in a fixed bed of MRs development. catalyst in order to by-pass the flammability area, to Among the number of membrane concepts, structure optimize the 02 profile concentration along the reactor, and and materials which have been developed for MR to maximize the selectivity in the desired oxygenate product.

Piles a combustible et interfaces pour les transports 60 The scenario occurring in a tubular membrane reactor red/ox characteristics of the gas phase (i.e. by the alkane/02 involving a membrane with a uniform thickness and ratio in the reactor). The chemical valve membrane prepared permeability, is schematized in figure 8a. The progressive by the infiltration method is very stable under red/ox cycling alkane consumption from the entrance to the exit of the conditions and a working temperature up to 600°C. This reactor continuously decreases the alkane/02 ratio. This innovative membrane concept could help in optimizing the phenomena is enhanced by the influence of the pressure drop alkane/02 ratio by an auto-regulation of the 02 concentration generated by the catalyst bed. which tends to increase the 02 all along the reactor. It could also limit the effects of hot­ permeation rate near the end of the reactor, due to the higher spots and then improve the reactor efficiency [23]. pressure gradient in that zone [21], The important evolution of the aikane/02 ratio along the reactor, which is 4.2.3. Nanophase membrane materials for enhanced consequently not always optimized, alters the catalyst surface oxygen transport efficiency and decreases the reaction yield. Current development of CICM can benefit by recent advances in material science, namely nanophase materials with a number of properties unique to these systems. Catalyst bed Nanophase ceramics are inorganic materials with typical microstructural dimensions of less than 100 nm. When the ceramic is formed of nanocrystallites of less than 10 nm, surface atoms become preponderant in the structure, yielding exacerbated surface adsorption and diffusion as well as catalytic properties towards gas phases. In other respects, arrangement of metal or semiconductor nanoparticles in the sub-10 nm range exhibit specific properties. In particular, a dispersion of noble metal nanoparticles at the surface of solid electrolyte grains are known to enhance the oxygen exchange rate of the material Classical inert membrane by influencing the oxygen adsorption equilibrium at the surface of the electrolyte. We have recently experienced the synthesis, by the sol- gel process, of nanophase Ce02 based materials [24]. Alumina was used as a dispersing and stabilizing matrix for Ce02 crystallites. Pd particles were dispersed in the material via a molecular metal precursor introduced at the sol stage. In specific conditions, the method leads to a nanophase material containing 4 nm ceria crystallites stable up to 700°C and Pd nanoparticles both embedded in an amorphous alumina matrix. The extremely high oxygen mobility observed by H2-TPR (Temperature Programmed Reduction) for this type of material in comparison with a material containing 10 nm ceria crystallites (figure 9) was attributed to the number of surface atoms : 30-60% for the crystallites of 5 nm compared to 15-30% for crystallites of 10 nm. A comparative study of a series of membranes for Figure 8 : Schematic representation of thechemical valve oxygen transport, prepared from these materials, is membrane concept compared to a conventional membrane underway in our laboratory. distributor for partial oxidation of alkanes [22], 2.6 Sol-gel derived material The proposed concept to overcome this problem, shown t in figure 8b, is based on the use of a "chemical valve ao 2.2 with mean Ce02 crystallite "o size = 4 nm membrane" whose permeability could be controlled by the E 1.8 red/ox characteristics of the gas phase. V205 was found to be an attractive key constituent for this membrane because of its § 1-4 4 Conventional material with reversible red/ox behavior (V20;/V203) and related textural 1.0 - mean Ce02 crystallite size variations able to regulate the membrane permeance [22]. = 12 nm We have shown that the membrane permeability can be c 0.6 controlled by the red/ox characteristics of the gas phase. The o (Jr" influence of the reducing gas on the evolution of membrane 0.2 :A< —I------1— —t— —r— -r T permeance was investigated in relation with the membrane 100 200 300 400 500 600 700 weight changes, up to 500°C. The red/ox kinetics have been Temperature (C) compared using a series of reducing gases (/-C4 Hi0, C3H8, C2H6, C2H4 , CH4 and H2). Results clearly showed that the Figure 9 : H2-TPR diagrams of sol-gel derived Ce02- permeability of such a membrane, when used as an 02 Af03-CaO based materials. Influence of Ce02 crystallite distributor in a reactor configuration, is controlled by the size on oxygen storage capacity [24],

Piles a combustible et interfaces pour les transports 61 [11] TORI, H.YAMAMURA, H.OGINO, 5. CONCLUSION H.KOBAYASHI, T.MITAMURA, J. Am. Ceram. Soc. 77J10|(1994) p2771 Very promising pathways are under investigation with [12] P.S.MAIYA, T.J.ANDERSON, R.L.MIEV1LLE, MRs which could help to improve considerably the J.T.DUSEK, J.J.P1CCIOLO, U.BALACHANDRAN, production of hydrogen for fuel cells. This has been Maximizing H2 production by combined partial possible thank to the application of innovative concepts oxidation of CH4 and water gas shift reaction. Applied arising from material science, catalysis and chemical catalysis A: General 196 (2000) pp65-72 engineering. Some very promising catalyst and/or [13] Y.M.LIN, M.H.REI, Process development for membrane materials have been already identified. Several generating high purely hydrogen by using supported of them have been studied in details and tested for their palladium membrane as steam reformer. Int. J. thermo-chemical and integrity upon use and aging in MRs. Hydrogen Energy, in press (1999) Results show that the synthesis and shaping methods [14] A.JULBE, D.FARRUSSENG, C.GUIZARD, Porous greatly influence the performance and stability of these ceramic membranes for catalytic reactors, overview and materials. Then, beside the very active research for new new ideas, J. Membrane Sci. in press catalysts and membranes, an R&D effort put on new [15] C. GUIZARD, A. JULBE, Nanophase ceramic ion structure formation (typically nanostructures) and shaping transport membranes for oxygen separation and oxygen methods of existing materials can greatly benefit to these gas stream enrichment. Recent Advances in Gas technologies. ' Separation by Microporous Membranes, N.Kanellopoulos (Ed), Membrane Science and 6. REFERENCES Technologies Series, 5, Elsevier, Amsterdam, 2000 [16] J.A.KILNER, Fast oxygen transport in acceptor doped [1] J.A.DAEMON, Catalytic Membrane Reactors, in: oxides, Solid State Ionics, 129 (2000) pp 13-23 Handbook of Heterogeneous Catalysis, G.Ertl, [17] R.HUGHES, Composite Pd membranes for catalytic H.Knozinger and J.Weitkamp (Eds), VCH Pub., 1997, membrane reactors, ICCMR, Zaragoza. 3-5 jul. 2000, Chapter 9.3 pp9-11. [2] H.P.HSIEH, Inorganic membranes for separation and [18] X.W.QI, Y.S.LIN, Electrical conducting properties of reaction, Membrane Science and Technology Series, 3, proton conducting terbium-doped strontium cerate Elsevier, Amsterdam, 1996. membranes, Solid State Ionics, 120(1-4] (1999) p85 [3] D.W.SCHAEFER, Engineered porous materials. MRS [19] A.-K.PRABHU, S.TED OYAMA, Highly hydrogen Bulletin 9|4] (1994) pl4 selective ceramic membranes: application to the [4] U.BALACHADRAN, J.GUAN, S.E.DORRJS, AC. transformation of greenhouse gases, J. Membrane Sci., BOSE, G.J.STIEGEL, Development of mixed- 176 (2000)pp233-248 conducting dense ceramic membranes for hydrogen [20] A.JULBE, D.FARRUSSENG, C.GUIZARD, separation. ICIM4 (Inorganic membranes)- July 1996, J.C.JALIBERT, C.MIRODATOS Characteristics and D.E. Fain (Ed), Gatlinburg-Tennessee-USA, pp:44 1-450 performances in the oxidative dehydrogenation of [5] A.J.BURGGRAAF, Transport and separation properties propane of a catalytically active V-MFI zeolite of membranes with gases and vapors. Fundamentals of membrane, Catalysis Today 56 [1-3] (2000) pp 199-209 Inorganic Membrane Science and Technology, A.J. [21] J.N.ARMOR, Membrane catalysis: where is it now, Burggraaf and L. Cot (Eds), Membrane Science and what needs to be done? Catalysis Today 25 (1995) pi99 Technology Series, 4, Elsevier, Amsterdam, 1996, [22] D.FARRUSSENG, A.JULBE, C.GUIZARD, Chapter 9 Synthesis and characterisation of a vanadium based [6] S.IRUSTA, M.P.PINA, M.MENENDEZ, "chemical valve" membrane. Submitted to Separation J. SANTAMARJA, Development and application of and Purification Technology perovskite-based catalytic membrane reactor. Catalysis [23] A.JULBE, D.FARRUSSENG, D.COT, C.GUIZARD, letters 54 (1998) p69. The chemical valve membrane: a new concept for an [7] C.LANGE, S.STORCK, B.TESCHE, W.F.MAIER, auto-regulation of 02 distribution in membrane reactors. Selective hydrogenation reactions with a microporous Submitted to Catalysis Today membrane catalyst prepared by sol-gel dip-coating. J. of [24] A.JULBE, L.DALMAZIO, D.COT, C.GUIZARD, Catalysis 175[2] (1998) p280 R.BERJOAN, C.KIELY, A.BURROWS, [8] M.P.HAROLD, C.LEE, A.J.BURGGRAAF, M PIRJAMALI, Evaluation of sol-gel methods for the K. KEIZER, V.T.ZASPALIS, R.S.A.DE LANGE, synthesis of Ce02 based materials with enhanced Catalysis with inorganic membranes. MRS Bulletin 9[4] oxygen mobility. (In preparation) (1994) p44 [9] A.DICKS, J.LARMINIE, Reforming of fossil fuels. 4 th European Solid Oxide Fuel Cell Forum, Lucerne. 10-14 jul. 2000, pp927-936 [10] S.HAMAKAWA, T.HAYAKAWA, K.SUZUKI, K. MURATA, K.TAKEHIRA. ICIM5 (Inorganic Membranes), Nagoya. Jun.1998 p350

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